The dermal denticles or "shagreen" of the ordinary Sharks and Dog-Fishes (Elasmobranchii) probably represent the most primitive form of exoskeleton. In the development of a dermal denticle a papilla of the dermis grows up into the overlying epidermis, pushing before it the basal layer of epidermic cells, which forms an investment to the papilla and constitutes the so-called "enamel organ" (Fig. 100). The papilla itself subsequently becomes converted into dentine, leaving, however, a central pulp-cavity, while the apex of the papilla is invested by a cap of enamel formed by the enamel organ. Ultimately the base of the papilla widens out into a more or less rhomboidal basal plate formed of bone. In this way there is formed a {184}pointed, enamel-tipped spine of dentine which protrudes through the epidermis, and projects backwards on the surface of the body, but is firmly fixed in the skin by the basal plate with which it is continuous. The centre of the under surface of the basal plate is perforated for the entrance of the blood-vessels which pass to the cellular pulp in the axis of the spine. In the adult Fish the denticles form a fairly close-set covering to the whole body, including the head and even the surfaces of the fins, and are larger on the dorsal than on the ventral surface (Fig. 99). In the Rays (_Raia_) they are more sparsely scattered, and in different parts of the body may form spines of considerable size for offensive or defensive purposes. The spines vary greatly in shape in different members of the group, sometimes being acutely pointed, and sometimes flattened or depressed, and often they are furnished with smaller accessory spines developed at their bases or from the surface of the basal plate. An arrangement of the denticles in oblique transverse rows is observable in some genera (e.g. _Scyllium_). In the Saw-Fishes (e.g. _Pristis_) the denticles which fringe the lateral margins of the long flattened rostrum are not only enormously enlarged, but are implanted in sockets and form the teeth of the saw (Fig. 262). In the Holocephali the smooth skin is almost entirely devoid of exoskeletal structures, but dermal denticles are present on the frontal and anterior claspers, and in the young there may be a double row of small denticles along the back. [Illustration: FIG. 99.—Surface view of the dermal denticles of _Scyllium_ sp., showing their arrangement in oblique transverse rows. _b_, Basal plate; _c_, canal which perforates the basal plate and becomes the axial pulp-cavity of the spine; _f.b_, intersecting fibrous bands of the dermis; _s_, spine; in the spine of one scale the dentinal tubules are shown. The smaller denticles are those most recently formed. (After Klaatsch.)] {185}[Illustration: FIG. 100.—Vertical section through the skin of an embryo Shark. _C_, Dermis; _c.c.c.d_, layers of the dermis; _E_, epidermis; _e_, enamel organ; _o_, enamel layer; _p_, papilla of the dermis. (From Wiedersheim, after Gegenbaur.)]